The invention relates to fuel cells.
Fuel cells are devices that convert chemical energy stored in a fuel into electrical energy by combining the fuel with an oxidant. In most commercial fuel cells, a hydrogen fuel is oxidized to generate an electric current, and then combined with oxygen to form water.
A typical fuel cell includes two electrodes, an anode and a cathode, separated by an electrolyte. Diatomic hydrogen molecules are provided to the anode, while an oxygen source (usually ambient air) is provided to the cathode. At the anode, electrons are stripped from the hydrogen molecules by an oxidation reaction and diverted, producing an electric current that can be used to perform useful work. The hydrogen ions that result from the oxidation reaction travel through the electrolyte and react with reduced oxygen molecules, forming water. The chemical reactions that occur at the anode and cathode can be described by the following two equations:
2H2xe2x86x924H++4exe2x88x92 at the anode, and
O2+4H++4exe2x88x92xe2x86x922H2O at the cathode.
The oxidation reaction at the anode is typically assisted by a catalyst, such as platinum.
In most existing fuel cell systems, the hydrogen molecules provided to the anode are derived from fossil fuels, such as natural gas, in a process called reformation. In the reformation process, a fossil fuel is combined with water at high temperatures to produce a xe2x80x9creformate.xe2x80x9d If the fossil fuel is natural gas (which is mostly methane), then the primary reformation reaction will be:
CH4+2H2O+heatxe2x86x92CO2+4H2
In addition to producing carbon dioxide and hydrogen, however, the reformation process also produces carbon monoxide by the following competing reaction:
CH4+H2O+heatxe2x86x92CO+3H2
Before delivering the reformate to the anode, the amount of carbon monoxide in the reformate can be reduced by a shift converter, which combines carbon monoxide with water to form carbon dioxide and additional hydrogen:
CO+H2Oxe2x86x92CO2+H2.
For fossil fuels that include larger hydrocarbons, the reformation process may also include pyrolysis, or xe2x80x9cthermal cracking.xe2x80x9d
Fuel cells are usually classified by the type of electrolyte used to separate the anode from the cathode. One common type of fuel cell is the proton exchange membrane (PEM) cell, in which the electrolyte is a thin, solid polymer matrix that is permeable to hydrogen ions but poses at least a partial barrier to the hydrogen fuel. A typical PEM cell includes an anode side flow plate, a cathode side flow plate, two gas diffusion layers (GDLs) disposed between the flow plates, and a membrane-electrode assembly (MEA) between the GDLs. The MEA includes the membrane and a catalyst, e.g., platinum. The catalyst can be formed, for example, as part of a slurry that bonds the GDLs to the membrane, or as an ink-slurry added separately to the membrane. The GDLs can be made, for example, from carbon cloth.
Since each fuel cell typically generates a relatively small voltage (e.g., less than 1 volt), cells are often combined in series to form a fuel cell stack. A fuel cell stack typically includes MEAs placed between distribution flow plates, and manifolds connecting the flow plates. The hydrogen source (e.g., the reformate) and the oxygen source (e.g., ambient air) are fed into a manifold supply passage, and distributed to the electrodes by the flow plates. Water produced by the fuel cells can be drained through a manifold discharge passageway.
Both fuel cells and fuel cell stack structures are described in greater detail in Meacher et al., U.S. Pat. No. 5,858,569, which is incorporated herein by reference.
In general, in one aspect, the invention features a method of removing a contaminant from a fuel cell. The method includes passing a removal substance through a flow channel in the cell, where the substance is selected to remove the contaminant.
Embodiments of this aspect of the invention may include one or more of the following features. The substance is selected to remove a metallic ion, SOx, NOx, or an organic molecule. If the contaminant is a metallic ion, the substance can be, e.g., an acidic solution or a chelating agent. If the contaminant is an organic molecule, then the substance can be an oxidant, such as an organic solvent, a peroxide, ozone, or ozonated water. If the contaminant is SOx, then the substance can be, e.g., an alkaline solution. The substance can be either a gas or a liquid.
The substance can be passed through the flow channel by passing the substance into a flow inlet, where the flow inlet connects to the flow channel. The substance can then pass through the flow channel and exit through a flow outlet. If the fuel cell has a plurality of flow channels, where each channel has a flow inlet and a flow outlet, the method can include passing the substance through each flow inlet.
In another aspect, the invention features a method of identifying contaminants present in a fuel cell that has a flow inlet, a flow outlet, and a flow channel that connects the inlet to the outlet. The method includes: (a) passing a wash solution, e.g., water or a water-acetone solution, into the inlet such that it passes through the flow channel; and (b) analyzing the wash solution after it exits the flow outlet, using, e.g., chromatography, mass spectrometry, or chemical analysis, to identify contaminants present in the cell.
In another aspect, the invention features a method of decontaminating a fuel cell that has a flow inlet, a flow outlet, and a flow channel that connects the inlet to the outlet. The method includes: (a) passing a wash solution into the inlet such that it passes through the flow channel; (b) analyzing the wash solution after it exits the outlet to identify a contaminant present in the cell; (c) selecting a removal substance targeted to remove the contaminant; and (d) flushing the removal substance through the flow channel.
Embodiments of this aspect of the invention may include one or more of the following features. The method can further include passing a second wash solution into the inlet such that it passes through the flow channel after completion of the flushing step, and then analyzing the second wash solution after it exits the outlet to determine if the flushing step removed the contaminant. The analyzing step can include identifying a plurality of contaminants present in the fuel cell, the selecting step can includes selecting a plurality of removal substances targeted to remove the respective contaminants, and the flushing step can include flushing the plurality of removal substances through the flow channel in succession.
In addition, the method can further include selecting a second removal substance targeted to remove the contaminant and flushing the second removal substance through the flow channel.
In another aspect, the invention features a method of decontaminating a fuel cell stack that has a fuel entrance, a fuel delivery manifold, and a discharge manifold. The method includes: (a) passing a wash solution into the fuel entrance such that it passes through the fuel delivery manifold, into fuel cells within the fuel cell stack, and out the discharge manifold; (b) analyzing the wash solution after it exits the discharge manifold to identify a contaminant present in the stack; (c) selecting a removal substance targeted to remove the contaminant; and (d) passing the removal substance into the fuel entrance.
Embodiments of this aspect of the invention may include one or more of the following features. The removal substance can be a liquid, and the passing step can include flushing a volume of the liquid into the fuel entrance, where the liquid volume is greater than an amount of space within the stack available for liquid. If the cell includes an oxygen delivery manifold, then the method can further include passing the removal substance into an oxygen entrance.
In another aspect, the invention features a kit for decontaminating a fuel cell stack. The kit includes an element for identifying a contaminant in the stack, and a removal substance targeted to remove the contaminant when the substance is flushed through the stack.
Embodiments of this aspect of the invention may include one or more of the following features. The element can identify a plurality of contaminants in the stack, and the kit can include a plurality of removal substances targeted to remove the respective contaminants when flushed through the stack. The element can include a chromatography device.
As used herein, the term xe2x80x9cinletxe2x80x9d or xe2x80x9cflow inletxe2x80x9d means any orifice capable of admitting a gas or fluid to a flow channel.
Embodiments of the invention may include one or more of the following advantages. Embodiments of the methods and kits allow removal of a variety of contaminants from fuel cells.
The methods and kits can enhance performance of a fuel cell or a fuel cell stack.
The removal substances and wash solutions are generally inexpensive and easily obtainable.
Other embodiments and advantages will be apparent from the following description and from the claims.